1. Field of the Invention
The present invention relates to a thin-film semiconductor having crystallinity and also to a method of fabricating such a thin-film semiconductor. Furthermore, the invention relates to a semiconductor device utilizing such a thin-film semiconductor. In addition, the invention relates to a method of fabricating this semiconductor device.
2. Description of the Related Art
Techniques for forming a crystalline silicon film on a glass substrate or quartz substrate and fabricating thin-film transistors (TFTs) from this silicon film are known.
These TFTs are known as high-temperature polysilicon TFTs or low-temperature polysilicon TFTs.
High-temperature polysilicon TFT fabrication is a technique utilizing a heat treatment conducted at relatively high-temperatures such as 800xc2x0 C., 900xc2x0 C., and more, in fabricating crystalline silicon films. It can be said that this technique has derived from IC fabrication processes making use of single-crystal silicon wafers.
Of course, quartz substrates withstanding the above-described heating temperatures are used as substrates on which high-temperature polysilicon TFTs are formed.
On the other hand, low-temperature polysilicon TFTs are fabricated on cheaper glass substrates which are, of course, inferior in heat resistance to quartz substrates.
When a crystalline silicon film forming low-temperature polysilicon TFTs is fabricated, a heat treatment at a temperature lower than 600xc2x0 C. is used so that the glass substrate can stand up to it, or laser annealing technique which hardly thermally damages the glass substrate is exploited.
High-temperature polysilicon TFT fabrication technology is characterized in that it can integrate TFTs having uniform characteristics on a substrate at a high density.
On the other hand, low-temperature polysilicon TFTs can make use of glass substrates which are cheap and can easily provide large areas.
It is to be noted that with the current technology, high-temperature polysilicon TFTs do not differ greatly from low-temperature polysilicon TFTs in characteristics. The slight differences between them are that high-temperature polysilicon TFTs are superior in production yield and uniformity of characteristics across the substrate while low-temperature polysilicon TFTs are superior in productivity and production cost.
With respect to the characteristics, both kinds of TFTs show mobilities of approximately 50 to 100 cm2/ V s and S values of about 200 to 400 mV/dec (VD=1 V).
These characteristics mean that these TFTs can operate at speeds about two orders of magnitude higher than those of TFTs using amorphous silicon. However, the characteristics of the high-temperature polysilicon TFTs are much inferior to those of MOS transistors using single-crystal silicon wafers. Generally, the S values of MOS transistors employing single-crystal silicon wafers are on the order of 60 to 70 mV/dec. Furthermore, their operating frequencies are 1-2 orders of magnitude higher than those of high- and low-temperature polysilicon TFTs.
At present, TFTs are used to integrate the active matrix circuit of an active matrix liquid crystal display and its peripheral driver circuit on the same substrate. That is, the active matrix circuit and peripheral driver circuit are fabricated from TFTs on the same substrate.
In this configuration, the source driver circuit of the peripheral driver circuit is required to operate considerably above 10 MHz. Today, however, with respect to a circuit composed of high- and low-temperature polysilicon TFTs, a margin of only several megahertz is given to the operating speed.
Accordingly, it is customary to multiplex the operation, so that a matrix-driven liquid crystal display is fabricated. However, this method has the disadvantage that stripes appear on the viewing screen due to subtle deviations of timing of multiplexing.
A conceivable forthcoming technique is to integrate oscillator circuits, D/A converters, A/D converters, and digital circuits for performing various kinds of image processing on the same substrate, in addition to the peripheral driver circuit including a shift register circuit and a buffer circuit.
However, it is necessary that the aforementioned oscillator circuits, D/A converters, A/D converters, and digital circuits for performing various kinds of image processing operate at higher frequencies than the peripheral driver circuit.
Therefore, it is substantially impossible to fabricate these circuits from the high- and low-temperature polysilicon TFTs obtained by the current techniques.
It is to be noted that an integrated circuit which consists of MOS transistors using a single-crystal silicon wafer and can be operated above 100 MHz has been put into practical use.
The invention disclosed herein is intended to provide TFTs capable of building a circuit which can be operated at the above-described high speeds (generally, more than tens of megahertz).
It is another object of the invention to provide TFTs having characteristics comparable to those of MOS transistors fabricated, using a single-crystal silicon wafer. It is a further object of the invention to provide means for fabricating these TFTs. It is a yet other object of the invention to provide a semiconductor device to which requisite functions are imparted by TFTs having such excellent characteristics.
A semiconductor device according to the present invention comprises a plurality of TFTs formed on a substrate having an insulating surface. The active layer of the TFTs is formed by a crystalline silicon film. This crystalline silicon film is formed by making use of crystals grown radially from a multiplicity of points.
This structure is obtained where the TFTs are fabricated, using the crystalline silicon film grown into morphologies shown in FIGS. 3 and 6.
Examples of substrates having insulating surfaces include glass substrates (which are required to withstand the process temperature), quartz substrates, and semiconductor substrates having insulating films formed thereon.
The above-described crystalline silicon film consisting of crystals grown radially from a multiplicity of points can be obtained by performing crystallization step, formation of a thermal oxide film, and removal of the thermal oxide film. The aforementioned crystallization is carried out by a heat treatment, using a metallic element (typified by nickel as described later) that promotes crystallization of silicon. The thermal oxide film described above is formed in an oxidizing ambient containing a halogen element.
Another semiconductor device according to the invention also comprises a plurality of TFTs formed on a substrate having an insulating surface. The active layer of the TFTs is formed by a crystalline silicon film. This crystalline silicon film is composed of a multiplicity of elongated crystalline structures grown in a certain direction. The widths of these crystalline structures range from dimensions comparable to the film thickness to 2000 xc3x85. The certain direction differs among the individual TFTs.
More specifically, where a number of TFTs are manufactured, using the crystalline silicon film grown into the crystal morphologies shown in FIGS. 3 and 6, the crystal growth direction (the direction of anisotropy of the elongated structures) in the active layer forming the TFTs differs among the individual TFTs.
Of course, some TFTs have active layers having the same crystal growth direction but most of the TFTs adopt the above-described structure.
For example, if an active matrix circuit is made of a crystalline silicon film consisting of crystals grown into a morphology as shown in FIG. 3, numerous TFTs arranged in hundreds of devices x hundreds of devices achieve the above-described structure.
The crystalline silicon film used in the present invention disclosed herein consists of crystals which are continuous with each other in a certain direction, as shown in FIG. 8. These successive elongated crystal structures have widths ranging from dimensions comparable to the film thickness to about 2000 xc3x85. These numerous crystal structures form regions which extend substantially parallel on opposite sides of grain boundaries.
Macroscopically, these crystal structures extend radially as shown in FIGS. 7 and 6.
A method according to the present invention comprises the steps of: forming an amorphous silicon film on an insulating surface; crystallizing the amorphous silicon film by the action of a metallic element that promotes crystallization of silicon to obtain a crystalline silicon film; performing thermal processing at 800-1100xc2x0 C. in an oxidizing ambient containing a halogen element to form a first thermal oxide film on a surface of the crystalline silicon film; removing the first thermal oxide film; and forming a second thermal oxide film on the surface of the crystalline silicon film, whereby obtaining a final crystalline silicon film consisting of crystals grown radially from a multiplicity of points.
In the above-described method, in order to improve the quality of the final crystalline silicon film, it is important to make the total thickness of the first and second thermal oxide films greater than the thickness of the final crystalline silicon film.
This is because the formed thermal oxide films drastically improve the quality of the crystalline silicon film.
Nickel is used quite advantageously as the metallic element for promoting crystallization of silicon in terms of reproducibility and effects. Generally, one or more elements selected from the group consisting of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, and Au can be used as this metallic element.
Where nickel element is employed, the concentration of nickel finally remaining in the silicon film is approximately 1xc3x971014 to 5xc3x971018 atoms/cm3. If the gettering conditions for the thermal oxide films are optimized, the upper limit of this concentration can be reduced to about 5xc3x971017 atoms/cm3. The concentration can be measured by SIMS (secondary ion mass spectroscopy).
Generally, the lower limit of the nickel concentration is approximately 1xc3x971016 atoms/cm3. That is, this amount of nickel is left behind because it is normally difficult to remove the effects of nickel adhering to the substrate or equipment if the cost is also taken into account. However, the concentration of the residues can be reduced further by cleaning the equipment to a higher level or improving the manufacturing processes.
Accordingly, where general manufacturing processes are adopted, the concentration of the remaining nickel element is about 1xc3x971016 to 5xc3x971017 atoms/cm3.
During fabrication of a thermal oxide film, the metallic element moves into the thermal oxide film and so the concentration of nickel element in the direction of thickness of the obtained crystalline silicon film has a gradient or distribution.
Generally, it is observed that the concentration of the metallic element in the crystalline silicon film tends to increase toward the interface at which the thermal oxide film is formed. Furthermore, it is observed that depending on the conditions, the concentration of the metallic element tends to increase toward the substrate or buffer layer, i.e., toward the interface on the back side.
Where a halogen element is added to the ambient when a thermal oxide film is formed, this halogen element shows a concentration distribution similar to that of the metallic element. That is, the concentration distribution is such that the concentration increases toward the front surface and/or rear surface of the crystalline silicon film.
The thickness of the final crystalline silicon film according to the present invention is preferably set to 100 to 750 xc3x85, more preferably 150 to 450 xc3x85. By selecting the film thickness in this way, the crystalline structure shown in FIGS. 6-8 can be obtained more clearly and with improved reproducibility.
It is necessary that the thickness of the final crystalline silicon film be determined by taking account of the fact that the film thickness is reduced by the deposition of the thermal oxide film.
The crystalline silicon film described herein can be obtained by adopting the manufacturing steps described above. Furthermore, MOS TFTs utilizing the special features of the crystalline structure can be obtained.
Examples of the method of introducing the metallic element include application of a solution containing this metallic element, a method using a CVD process, methods relying on sputtering or deposition, a plasma processing method using an electrode containing this metal, and a method making use of gas adsorption.
A method of introducing a halogen element can use a means for adding HCl, HF, HBr, Cl2, F2, Br2, or CF4 to an oxidizing ambient such as oxygen ambient.
Furthermore, when the thermal oxide film is fabricated, if hydrogen gas is also introduced into the ambient to make use of the action of wet oxidation, then desirable results arise.
The temperature at which the thermal oxide film is grown is quite important. If one attempts to obtain a TFT which can be operated by itself at tens of megahertz or more and shows an S value of less than 100 mV/dec as described later, then it is necessary to set the heating temperature used during the formation of the thermal oxide film above 800xc2x0 C., more preferably 900xc2x0 C. or above.
The upper limit of this heating temperature should be set to about 1100xc2x0 C. which is the maximum processing temperature of quartz substrates.
The present invention lies in a technique for crystallizing an amorphous silicon film by means of heating to obtain a crystalline silicon film. This technique is characterized in that thermal processing is performed while holding nickel element in contact with the surface of the amorphous silicon film, thus giving rise to the crystalline silicon film. A thermal oxide film is formed on the surface of this crystalline silicon film by performing thermal processing at 800-1100xc2x0 C. in an oxidizing ambient containing a halogenic element.
Thus, a crystalline silicon film grown into the peculiar crystalline state as shown in FIGS. 6-8 can be obtained.